While the two kinds of sexual glands are usually located in different parts of the generating organism, there are, nevertheless, a few cases in which the sex-cells are formed directly and together from one and the same gland. These glands are called hermaphroditic glands. Such structures are very notable in several highly differentiated groups of the metazoa, and have clearly been developed from gonochoristic structures in lower forms. The class of crested medusæ, or ribbed medusæ (ctenophoræ), contains glasslike, sea-dwelling cnidaria of a peculiar and complicated build, which probably descend from hydromedusæ (or craspedota). But whereas the latter have very simple gonochoristic structures (four or eight monosexual glands in the course of the radial canals or in the gastric wall), in the ctenophoræ the eight hermaphroditic canals run in a meridian arch from one pole of the cucumber-shaped body to the other. Each canal corresponds to a ciliary streamer, and forms ovaries at one border and testicles at the other; and these are so arranged that the eight intercostal fields (the spaces between the eight streamers) are alternately male and female. Still more curious are the hermaphroditic glands of the highly organized, land-dwelling, and air-breathing lung-snails (pulmonata), to which our common garden snail (arion) and vineyard snail (helix) belong. Here we have a hermaphroditic gland with a number of tubes, each of which forms ovaries in its outer part and sperma in the inner. Still the two kinds of sex-cells lead separately outward.
In most of the lower and aquatic histona both kinds of sex-cells, when they are ripe, fall directly into the water, and come together there. But in most of the higher, and especially the terrestrial, organisms special exits or conducting canals have been formed for the sex-products, the sexual ducts (gonoductus); in the metazoa the female have the general name of oviducts and the male spermaducts (or vasa deferentia). In the viviparous histona special canals serve for the conveyance of the sperm to the ovum, which remains inside the mother's body; such are the neck of the archegonium in the cryptogams, the pistil in the phanerogams, and the vagina in the metazoa. At the outer opening of these conducting canals special copulative organs are developed, as a rule.
When the ejected sex-cells do not directly encounter each other (as in many aquatic organisms), special structures have to be formed to convey the fertilizing sperm from the male to the female body. This process of copulation becomes important, as it is associated with characteristic feelings of pleasure, which may cause extreme psychic excitement; as sexual love it becomes, in man and the higher animals, one of the most powerful springs of vital activity. In many of the higher animals (namely, vertebrates, articulates, and mollusks) there are also formed a number of glands and other auxiliary organs which co-operate in the copulation.
The manifold and intimate relations which exist, in man and the higher animals (especially vertebrates and articulates), between their sexual life and their higher psychic activity, have given rise to plenty of "wonders of life." Wilhelm Bölsche has so ably described them in his famous and popular work, The Life of Love in Nature, that I need only refer the reader to it. I will only mention the great significance of what are called "secondary sexual characters." These characteristics of one sex that are wanting in the other, and that are not directly connected with the sexual organs—such as the man's beard, the woman's breasts, the lion's mane, or the goat's horns—have also an æsthetic interest; they have, as Darwin showed, been acquired by sexual selection, as weapons of the male in the struggle for the female, and vice versa. The feeling of beauty plays a great part in this, especially in birds and insects; the beautiful colors and forms which we admire in the male bird of paradise, the humming-bird, the pheasant, the butterfly, etc., have been formed by sexual selection (cf. the History of Creation).
In various groups of the histona the male sex has become superfluous in the course of time; the ovula develop without the need of fertilization. That is particularly the case in many of the platodes (trematodes) and articulates (crustacea and insects). In the bees we have the remarkable feature that it is only decided at the moment of laying the egg whether it is to be fertilized or not; in the one event a female and in the other a male bee is formed from it. When Siebold proved at Munich these facts of miraculous conception in various insects, he was visited by the Catholic archbishop of the city, who expressed his gratification that there was now a scientific explanation possible of the conception of the Virgin Mary. Siebold had, unfortunately, to point out to him that the inference from the parthenogenesis of the articulate to that of the vertebrate was not valid, and that all mammals, like all other vertebrates, reproduce exclusively from impregnated ova. We also find parthenogenesis among the metaphyta, as in the chara crinita among the algæ, the antennaria alpina and the alchemilla vulgaris among the flowering plants. We are, as yet, ignorant for the most part of the causes of this lapse of fertilization. Some light has been thrown on it, however, by recent chemical experiments (the effect of sugar and other water-absorbing solutions), in which we have succeeded in parthenogenetically developing unfertilized ova.
In the higher animals the complete maturity and development of the specific form are requisite for reproduction, but in many of the lower animals it has been observed recently that ovula and sperm-cells are even formed by the younger specimens in the larva stage. If impregnation takes place under these conditions, larvæ of the same form are born. And when these larvæ have afterwards reached maturity and reproduced in this form, we call the process dissogony ("double-generation"). It is found in many of the cnidaria, especially the medusæ. But if larvæ propagate by unfertilized ova, and so reproduce their kind parthenogenetically, the process is known as pædogenesis ("young-generation"). It is found particularly in the platodes (trematodes) and some of the insects (larvæ of cecidomyca and other flies).
In a large number of lower animals and plants sexual and asexual generation regularly alternate. Among the protists we find this alternation of generation in the sporozoa; among the metaphyta in the mosses and ferns; and among the metazoa in the cnidaria, platodes, tunicates, etc. Often the two generations differ considerably in shape and degree of organization. Thus, in the mosses the asexual generation is the spore-forming moss capsule (sporogonium), while the sexual is the moss plant with stalk and leaves (culmus). In the case of the ferns, on the other hand, the latter is spore-forming and monogenetic, while the thallus-formed, simple, and small fore-germ (prothallium) is sexually differentiated. In most of the cnidaria a small stationary polyp is developed out of the ovum of the free-swimming medusa, and this polyp, in turn, generates by budding medusæ, which reach sexual maturity. In the tunicates (salpa) a sexual social form alternates with an asexual solitary form; the chain-salpa of the former are smaller and differently shaped than the large individual salpa of the latter, which again generate chains by budding. This special form of metagenesis was the first to be observed, as it was in 1819 by the poet Chamisso, when he sailed round the world. In other cases (for instance, in the closely related doliolum) a sexual generation alternates with two (or more) neutral ones. The explanation of these various forms of alternating generations is given in the laws of latent heredity (atavism), division of labor, and metamorphosis, and especially by the biogenetic law.
While in real metagenesis (alternation of generations in the strict sense) the asexual generation propagates by budding or spore-formation, this is done parthenogenetically in the cognate process of heterogenesis. This it is which, especially in many of the articulates, causes an immense increase of the species in a short time. Among the insects we have the leaf-lice (aphides), and among the crustacea the water-fleas (daphnides), that propagate in great numbers during warm weather by unfertilized "summer-ova." It is not until the autumn that males appear and fertilize the large "winter-ova"; in the following spring the first parthenogenetic generation issues from the winter eggs. The two heterogenetic generations are very different in the parasitic suctorial worms (trematodes). From the fertilized ovum of the hermaphrodite distoma we get simply constructed nurses (pædogenetic larvæ), inside which cercaria are generated from unfertilized ova; these travel, and are afterwards converted (inside another animal) into distoma once more.
I have given (General Morphology, chap, ii., p. 104) the name of strophogenesis to the complicated process of cell-reproduction, which we find in the ontogeny of most of the higher histona, both phanerogams and cœlomaria. In these there is not a real alternation of generations, as the multicellular tissue-forming organism develops directly from the impregnated ovum. But the process resembles metagenesis in so far as the ontogenetic construction consists itself in a repeated division of the cells. Many generations of cells proceed by cleavage from the one stem-cell (the impregnated ovum) before two of these cells become sexually differentiated, and form a generation of sexual cells. However, the essential difference consists in the fact that all these generations of cells—in the body of both the higher animals and the flowering plants—remain joined together as parts of a single bion (a unified physiological individual); but in the alternation of generations each group produced is made up of a number of bionta, which live as independent forms—often so different from each other that they were formerly thought to be animals of separate classes, such as the polyps and medusæ. Hence we must not describe the reproductive circle of the phanerogams as an alternation of generations, although it has started from the fern (by abbreviated heredity).